Grid construction for radio tubes



v Dec. 17, 1940 l. E. MoURoMTsEl-'F 2,225,077

GRID CONSTRUCTION FOR RADIO TUBES Filed Jan. 28, 1958 1s1315 @w22 I mmm ` ATTORN EY Patented Dec. 17, 1940 UNITED ,STATES PATENT `OFFICE Westinghcuse'Electric & Manufacturing Company, East Pittsburgh,-1a`., a `corporationof Pennsylvania Application January 2s, 193s, scrialNo.1s7,475'

2 Claims.

The present invention relates to electron discharge ldevicesl provided with an anode, thermionic cathode and `a controlling grid and has particular reference to a tube of this type wherein secondary electron emission from the grid .is controlled.

Discharge devices of this general type are wellknown to the art `and it is a recognized phenomenon that thegrid electrode inherently gives off electrons due to bombardment by .the electrons flowing from the cathode. The electrons given off. by the grid are referred to as secondary'electrons which `are added to .the electrons emanating from the cathode and nally reaching the anode. In many instances this phenomenon is considered benecial because the secondary emission current'is. subtractedl in the external grid circuit from the primary electronic current to the grid, thusv eiiecting a vreduction in thegrid excitation power. required. At the same time the plate.current is in the same measure augmented by the stream of secondary electrons', as'above noted, resulting in `an increase of power'output from the device directly 'attributable/to secondary emission from .the grid.

Despite these beneficial' vproperties resulting from secondary emission, it all too frequently becomes a source of trouble, such as'to impair tube operation. For example; inthe usual circuits employing three electrode discharge :devices the detrimental effects' of secondary emission are `ordinarilymanifested in thextendency of the device to' oscillate at parasitic frequencies. In addition, in Class B amplifiers, emphasized secondary emission may also unfavorably-aiect the shape of the input Wave'to'the detriment of modulation quality, etc. v

This is particularly true when during operation the device has a -relatively high instantaneous plate voltage and low positive grid potential, for secondary emission in such instances is distinctly noticeable. When the anode dynamic characteristics of the device pass through the region of high plate voltage and low grid potential, the grid current reversesv during a portion'of the cycle which produces negative slopes in the grid dynamic characteristics, which accounts for such devices frequently losing control during operation or generating parasitic oscillations. The higher the D. C.l operating plate voltage, the more npronounced becomes ythe detrimental effect of secondary emission.

To obv-iate the detrimental eiects of secondary emission it is known in the' art to provide 'a grid of a non-electron-l emitting material or toi coat (Cl. Z50- 176) the grid with a material which prevents electron emission. For example, by making the grid of graphite or applying a graphite or carbon coating thereto, secondary emission may be suppressed to the point where` no reversal in grid current occurs during the entire operating cycle, evenI at very y high operating plate voltages. There accordingly results a complete elimination of the tendency to vigorousV parasitic oscillatio-ns. However, the grid excitation power required for a definite performance characteristic is much higher due tothe very absence of secondary emission.

It is' accordingly an object of the' present invention to provide an electron discharge" device 15 wherein secondary emission from the grid is so controlled as to enable an increaseV power output without attendant detrimental effects resulting from secondary emission.

Another object of the present invention is vthe provision of an electron discharge device wherein secondary emission from the gridI electrode is so' controlled that. all benefits inherently accruing therefrom 4are retainedwhile alldetrimental effects` of secondary emission are eliminated.

Anotherl object of the present invention isrthe provision of an electron discharge-device wherein the grid electrode is so constructed as to eliminate reversalJ in grid current and at the' same time requirerelativelyflow excitation power.

Another object of the present invention is'the provision of a gridsoconstructed as todistribute aT preselected electro-'static eld in the vicinity of the grdzsurface for controlling secondary'emis- Sion;

Still furtherobjects of the present invention will becomefobvious to those -skilledin the artby reference to the accompanying drawing wherein:

Fig. l is an elevational viewof anV electron dis'- charge device constructed in accordance with the presentinvention with parts -thereofbroken away to better illustrate the various parts.

Fig. 2: is'a sectional view on an enlarged scale taken onthe'line II-II of Fig. 1. 45 Fig. 3 isa cross-sectional .view taken on'the line III--III of Fig. 2 and' on an enlarged scale. Figfl is a cross-sectional lview'showing 'amodication which thegrid electrode may take.

Fig.' 5` is a cross-sectionalY View showing a further modification which the grid electrode maytake. l Fig. 6 is aflcrossesectio-nal'view taken on the line VI-VI-*oFig 5'.

Fig. 7. is a crosslsectional View showir'lgfanother modification which the grid electrode, as illustrated in Fig. 5, may take.

Fig. 8 is a cross-sectional view of a still further modification which the grid electrode may take.

Fig. 9 is a diagrammatic illustration of the electro-static field distribution and electron flow in a standard three-electrode discharge device when supplied with low plate and high grid potentials, and

Fig. 10 is a diagrammatic illustration of the electro-static field distribution and electron ow in a standard three-electrode discharge device when supplied with high plate and low grid tentials. y

Referring now to the drawing in detail the tube shown in Fig. 1 may comprise a vitreous envelope 5 provided with enlarged ends 6 and 'I having reentrant stem portions 8 and 9 through which leading-in conductors extend to various electrodes as is customary in the art. In the particular type tube shown, the anode I0 is of a suitable metal and forms the intermediate section of the envelope which is surrounded by a cooling jacket I2, through which a suitable cooling medium may be circulated, to dissipate heat from the anode during operation of the device.

The thermionic cathode I3 is in the form of a filament which may be supplied with suitable heating current and is disposed substantially concentric with respect to the surrounding anode I0.

Intermediate the anode I0 and cathode I3 is a control grid I4 which, when supplied with suitable potential, controls electron ow from the thermionic cathode to the anode. This grid I4 may be of any desired configuration, such for example as a substantially cylindrical member formed of wire mesh, rods, or discs, which produces an electro-static field inuencing the electrons normally flowing from the cathode in the direction of the anode.

The construction of the device thus far described is of the general type at present known to the art in which secondary emission from the grid results in parasitic oscillations, as before mentioned, so long as the grid is of a material which readily emits secondary electrons. On the other hand, if the grid is of a material, such as graphite, or coated with a non-electron emitting material, which entirely eliminates secondary emission, this results in somewhat lower outputs from the device with a higher required grid excitation power.

Since the present invention is concerned with the elimination of the disadvantages inherent in such prior art tubes due to secondary emission, while at the same time retaining the advantages, a brief consideration of the mechanism of the transfer of secondary electrons from the grid to the anode may assist toward a better understanding of the present invention.

Primarily, secondary emission is dependent upon the distribution of the electro-static field in the vicinity of the grid surface. Ordinarily the application of a positive potential to the anode of a discharge device causes the electro-static field to assume a simple distribution in the space between the anode and the filamentary cathode, and if the structures be cylindrical, the lines of force extend radially and equipotential surfaces are formed simulating concentric cylinders.

' Upon the inter-position of a grid and the application of a potential thereto this field will be distorted. However, when a positive potential is impressed upon the grid a value may be reached at which the original electrostatic field is restored, such value approximating from one-half to one-third the anode potential, and will be hereinafter referred to as the space potential.

By raising the grid voltage above its space potential the equipotential surfaces will bulge out in the direction of the anode, such as shown in Fig. 9 by the lines A, and in cases where the grid potential is below the space voltage the eld will protrude through the grid structure toward the filamentary cathode, as indicated in Fig. 10 by the lines A, with further distortion in either direction being dependent upon the degree of departure of the grid potential from the space voltage.

Referring now more specifically to Fig. 9 the positive grid potential is assumed to be in its apex during oscillation, and at such point in the grid potential wave the plate voltage falls to its lowest value. Due, however, to the shape of the electrostatic eld, the greater percentage of primary electrons striking the grid are confined to the rear surface of the grid facing the cathode, such as indicated by the dotted lines B of Fig. 9. The secondary electrons, which possess very low velocities and can overcome an adverse voltage of but a few volts ranging from approximately 8 to volts, are turned back by the adverse field gradient at the grid surface with the result that secondary emission results only from the front of the grid facing the anode, but is of such quantity as to not substantially raise the power output despite highest grid potential and highest primary grid current (electrons striking the grid) which together constitute high grid excitation.

In Fig. 10 the phenomenon resulting during the other portions ofthe oscillating cycle is depicted when the positive grid voltage is low. At this time the grid voltage falls below the space potential and the anode voltage rises to its highest value. Upon the occurrence of this condition, the anode field envelopes the grid on all sides with the result ythat a great many primary electrons emitted by the cathode are drawn to the anode, as indicated by the dotted lines B of Fig. 10. Also, due to this anode field, practically every secondary electron emitted by the grid is drawn to the anode, as indicated by the lines C of Fig. 10. Consequently, upon an excess in secondary electron emission over the primary electron emision, the grid current becomes negative and may lose control. Thus considering a full oscllating cycle, there is a period when what might be termed partial secondary emission oc.- curs, while during the remainder of the cycle an excess results, giving rise to vigorous parasitic oscillations or loss of control, as above noted.

Manifestly in order to eliminate secondary emission the tube must be operated at low voltages, in which case relatively few electrons are emitted by the grid; or the grid may be formed of a non-electron'emitting material such as graphite, in which case high voltages are permissible, but a high grid excitation energy is required. However, by proper design of the grid the disadvantages of secondary emission may be eliminated while the advantages thereof are retained. a

Referring now more specifically to Figs. 2 and 3, the grid electrode I4 of the tube of Fig. 1 `may be formed of a plurality of metallicuprights; or rods I5 supporting a plurality of graphite spacer rings or the like I6 making disc type grid structure. As caribe seen more readily in Fig. 3 these spacer rings are retained in spaced relation by means'of sleeves or the like I1 which surround the metallic rods I5. In order to eliminate secondary emission, these sleeves l1 may be formed of a semicircular portion of a material which is non-electron emitting and positioned on the rear surface of the grid facing the cathode i3. The remaining portion of the sleeves are then formed of a material which is high emissive, such as molybdenum or other similar metal and positioned on the front surface of the grid facing the anode l0.

To facilitate manufacture and assembly, the sleeves may be made entirely of molybdenum if desired and provided with a non-electron emitting semi-circular coating, such as graphite or carbon, on their rear surfaces, as shownin Fig. 7. Also the grid may be of the wire-Wound type in which the grid Wire is Wound about uprights and the surface of the wire as well as the uprights may be coated on their rear surfaces, as above mentioned.

In Fig. 4 a grid structure commonly referred to as the cage type is shown, and for the purpose of eliminating secondary emission from the rear surface, the upright rods forming the cylindrical cage may be grooved, as shown at I8, in lieu of being coated with a non-electron emitting surface. By attening the wire in a wirewound grid, as well as the stays as shown at I9 in Figs. and 6, or making the stays channel shaped as shown at 20 in Fig. 8, the same phenomenon results. the conguration of the rear surface of the grid, a great majority of secondary electrons emitted are prevented from reaching the adjacent anode eld, thus permitting such field to inluence the secondary electrons emitted only from the front surface of the grid.

In each instance, due tov It accordingly becomes obvioustofthose skilled in the art that by constructing a discharge device with a grid as above described, secondary emission is controlled to such an extent that the tube may be operated at high voltages vwithout a tendency to generate oscillations at parasitic frequencies and without the grid losing control. Moreover, by'so controlling secondary emission the necessity for high grid excitation power is eliminated. Since sufficient secondary emission reaches the anode to permit an increased power output, smaller tubes from the standpoint of power input may be employed for the same class of work.

Although several embodiments of the present invention have been shown and described, it is to be understood that other modifications thereof may be made without departing from the spirit and scope of the appended claims.

I claim: i

1. A discharge device comprising an anode, a cathode and an intermediate grid having a carbon surface on its side disposed adjacent the cathode for preventing the emission of secondary electrons therefrom and a metallic surface on the side disposed adjacent the anode for the emission of secondary electrons.

2. A discharge device comprising an anode, a cathode and an intermediate grid, said grid having a plurailty of grid disks, supports for said grid disks and cylindrical spacers on said supports between the grid disks, said spacers havying a semicircular inner surface of carbon and a semicircular outer surface of metal.

ILIA E. MOROMTSEFF. 

